Calcium Metabolism physiology of hormone 2007

Transcription

1 Calcium Metabolism physiology of hormone 2007

2 Calcium metabolism What is the recommended daily intake? 1000mg What is the plasma concentration? mmol/L 2.6mmol/L How is calcium excreted? Kidneys mmol/24 hrs How are calcium levels regulated? PTH and vitamin D (+others)

3 Calcium homeostasis Three Calcitropic hormones PTH 1,25 (OH)2 CC CT Three Three targets Bone Gut Kidney Three bone cells Osteoblasts Osteocytes Osteoclasts

4 Calcium homeostasis Gut absorption 1,25 (OH) 2 CC PTH 1,25 (OH) 2 CC Bone resorption Plasma Ca++ CT PTH CT 1,25 (OH)2 CC Bone accretion Calciuria

5 Calcium physiology Blood Calcium is tightly regulated by: PRINCIPLE ORGAN SYSTEMS GUT, BONE, KIDNEYS HORMONES PARATHYROID HORMONE (PTH), VITAMIN D,calcetonin INTEGRATED PHYSIOLOGY OF ORGAN SYSTEMS AND HORMONES MAINTAIN BLOOD CALCIUM

6 CALCIUM PHYSIOLOGY: BLOOD CALCIUM Calcium flux into and out of blood IN FACTORS: 1- intestinal absorption 2- Bone resorption OUT FACTORS: 1- Renal Excretion 2- Bone formation (Ca INCORPATION INTO BONE) BALANCE BETWEEN IN AND OUT FACTORS Organ physiology of Gut, Bone and kidney Hormone function of PTH and vitamin D3

7 CALCIUM HOMEOSTASIS DIETARY CALCIUM THE ONLY IN DIETARY HABITS, SUPPLEMENTS INTESTINAL ABSORPTION ORGAN PHYSIOLOGY ENDOCRINE PHYSIOLOGY BLOOD CALCIUM KIDNEYS ORGAN PHYS. ENDOCRINE PHYS. URINE BONE ORGAN, ENDOCRINE THE PRINCIPLE OUT

8 Vitamin D (cholecalciferol( cholecalciferol) Sources of vit D Diet u.v. light on precursors in skin Normal daily requirement 400IU/day Target organs bone - increased Ca release gut - increased Ca absorption

9 VITAMIN D PHYSIOLOGY VITAMIN D IS A HORMONE BY CLASSIC CRITERIA: MADE IN ONE PLACE (OR SEQUENTIALLY SEVERAL PLACES!), AND ACTING IN OTHER PLACES. THIS DISTINGUISHES IT FROM OTHER CLASSIC VITAMINS, SUCH AS VITAMIN C, B VITAMINS, ETC., WHICH ACT AS COFACTORS IN BIOCHEMICAL REACTIONS.

10 VITAMIN D SYNTHESIS SKIN LIVER KIDNEY 7-DEHYDROCHOLESTEROL UV VITAMIN D 3 VITAMIN D 3 25-HYDROXYLASE 25(OH)VITAMIN D 25(OH)VITAMIN D 1α-HYDROXYLASE 1,25(OH) 2 VITAMIN D (ACTIVE METABOLITE) TISSUE-SPECIFIC VITAMIN D RESPONSES

11 VITAMIN D MECHANISM OF ACTION VIT D / VDR RNA POL 5 UNTRANSLATED REGION VITAMIN D RESPONSIVE GENE TRANSCRIPTION START SITE IN THE NUCLEUS

12 FUNCTION OF VITAMIN D TISSUE SPECIFICITY GUT STIMULATE TRANSPORT OF CALCIUM AND PHOSPHATE IN THE SMALL INTESTINE (PRINCIPALLY DUODENUM) BONE STIMULATE TERMINAL DIFFERENTIATION OF OSTEOCLASTS STIMULATE OSTEOBLASTS TO STIMULATE OSTEOCLASTS TO MOBILIZE CALCIUM PARATHYROID INHIBIT TRANSCRIPTION OF THE PTH GENE (FEEDBACK REGULATION)

13 The Parathyroids,, associated with the thyroid but comprising separate structures

14 PTH Physiological role Production related to plasma calcium levels Control of calcium levels target organs bone - increased Ca/PO4 release kidneys increased reabsorption of Ca increased excretion of PO4 gut - indirect increase in calcium reabs by stimulting activation of vitamin D metabolism

15 4. Parathyroid hormone and calcitonin regulate blood calcium level The four parathyroid glands are embedded in the surface of the thyroid gland.

16 They secrete parathyroid hormone (PTH), a peptide: Raises blood calcium levels. Secretion regulated by calcium in the blood. Causes osteoclasts to break down bone, releasing Ca 2+ into the blood. Stimulates the kidneys to reabsorb Ca 2+. Stimulates kidneys to convert vitamin D to its active form that to stimulate the uptake of Ca 2+ from food in the intestine. PTH and calcitonin are antagonistic hormones.

17 A lack of PTH causes hypoparathyroidism, a tetany: Calcium levels in the blood drop. There are convulsive contractions of the skeletal muscles.

18 Fig. 45.9

19 Parathyroid hormone (PTH) Physiology PTH FUNCTIONS to preserve normal blood calcium ( and phosphate) PTH STIMULATES BONE RESORPTION AND, THUS, INCREASES BLOOD CALCIUM PTH STIMULATES RENAL TUBULAR REABSORPTION OF CALCIUM, AND THUS, INCREASES BLOOD CALCIUM PTH STIMULATES RENAL 1α-HYDROXYLATION 1 OF 25(OH)VITAMIN D, THUS INDIRECTLY STIMULATING INTESTINAL ABSORPTION OF CALCIUM

20 CALCIUM, PTH, AND VITAMIN D FEEDBACK LOOPS BONE RESORPTION URINARY LOSS 1,25(OH) 2 D PRODUCTION SUPPRESS PTH RISING BLOOD Ca NORMAL BLOOD Ca BONE RESORPTION URINARY LOSS FALLING BLOOD Ca STIMULATE PTH 1,25(OH) 2 D PRODUCTION

21 Calcitonin Physiological role Levels increased when serum Ca >2.25mmol/L Target organs Bone - suppresses resorption Kidney - increases excretion

22 ORGAN PHYSIOLOGY AND CALCIUM METABOLISM THERE ARE 3 PRICIPLE TISSUES THAT FUNCTION PROMINENTLY IN CALCIUM HOMEOSTATIS.. DISORDERS OF THESE TISSUES, OR OF THE CALCIOTROPIC FACTORS THAT AFFECT THEIR FUNCTION MAY RESULT IN DISORDERS OF CALCIUM METABOLISM INTESTINES KIDNEYS BONE

23 Factors affecting bone turnover Other hormones Oestrogen gut - increased absorption bone - decreased re-absorption Glucocorticoids gut - decrease absorption bone - increased re-absorption/decreased formation Thyroxine stimulates formation/resorption resorption net resorption

24 Skeletal System Bones are made of several tissues Primarily made of collagen and hydroxyapatite - Ca 10 (PO 4 ) 6 (OH) 2 Ca 10 About 206 bones in the human body

25 What is the composition of bone? The matrix 40% organic Type 1 collagen (tensile strength) Proteoglycans (compressive strength) Osteocalcin/Osteonectin Growth factors/cytokines/osteoid 60% inorganic Calcium hydroxyapatite The cells osteo-clast/blast/cyte/progenitor

26 Functions of Skeletal System SUPPORT: Hard framework that supports and anchors the soft organs of the body. PROTECTION: Surrounds organs such as the brain and spinal cord. MOVEMENT: Allows for muscle attachment therefore the bones are used as levers. STORAGE: Minerals and lipids are stored within bone material. BLOOD CELL FORMATION: The bone marrow is responsible for blood cell production.

27 Homeostatic Imbalances Rickets Disease of children due to a lack of vitamin D. Calcium is not deposited in bones. Bones become soft. Bowing of the bones, and other deformities occur.

28 Homeostatic Imbalances Osteomalacia Rickets of adults. Due to a lack of vitamin D. Calcium is not deposited in the bones. Bones become brittle.

29 Homeostatic Imbalances Osteoporosis Bone reabsorption is greater than bone deposition. Due to any of the following: Lack of estrogen in women. Lack of exercise to stress the bones. Inadequate intake of calcium and phosphorus. Abnormalities of vitamin D metabolism. Loss of muscle mass.

30 Risk factors for osteoporosis In-correctable Gender Race (non-black) Genetic predisposition Family history Correctable Smoking Alcoholism Caffeine Sedentary lifestyle Immobilization Excessive exercise Low BMI Gonadal failure Dietary lack Indoor life Drugs Heparin Corticosteroids

31 Osteoporosis

32 Vertebral bone Vertebral fracture Normal vertebrae

33 Osteoporotic fracture of femoral neck

34 Prevention of osteoporosis Diet Diet rich in calcium 1.2 g/d Vitamin Vitamin D 400 IU/d Exercise Exercise 40 min walk 44 times a week Sunshine Sunshine

35 Age Related Dysfunctions Arthritis: Osteoarthritis- 90% of pop. By age 40 chronic inflammation of articular cartilage can be normal age-dependent change can also be pathology due to? Age-related changes decrease blood supply trauma

36 Osteoarthritis

37 CAUSES OF HYPERCALCEMIA HORMONAL PRIMARY HYPERPARATHYROIDISM HYPERVITAMINOSIS D PARANEOPLASTIC (e.g., PTHrP, cytokines) NON-HORMONAL RENAL FAILURE DRUGS THIAZIDES, LITHIUM, OTHERS

38 Thank you